In general, fertilized eggs are used for influenza vaccine production. However, using fertilized eggs has the following disadvantages: chickens suitable for the production should be bred and managed; the influenza vaccine production should be adjusted to the production range of the fertilized eggs; and egg proteins cannot be completely eliminated which renders the vaccine unsuitable for inoculation to a person allergic to the egg proteins. Since long time ago, studies have been conducted on influenza vaccines produced by using cell cultures in order to replace the fertilized eggs that have the above disadvantages. Using cell cultures allows mass production in a short time owing to the indefinite supply of animal cells used for the cell culture. In addition, it has advantages in that the vaccine can be inoculated to a person allergic to egg proteins; and the influenza vaccines which cannot be isolated from fertilized eggs can be isolated from the cells. Furthermore, the viruses proliferated in cells show less gene mutations of viral antigens than in fertilized eggs.
Madin Darby canine kidney (hereinafter referred to as “MDCK”) cell lines, established from canine kidney, shows very strong surface adhesion; has been cultured using animal sera; and requires much space for mass production thereby necessitating high expenses. Also, adherent MDCK cell lines were reported to show high tumorigenicity.
Thus, there is a need to develop a novel MDCK cell line adapted to suspension cultures using a culture medium containing no animal-derived serum and proteins, which shows significantly lower tumorigenicity than conventional adherent MDCK.